Magnetic tape cassette having improved guide pole structure

ABSTRACT

A magnetic tape cassette having a guide pole which is in sliding contact with the magnetic surface of the tape produced in such a manner as to improve the running characteristics of the tape while being capable of being manufactured at a low cost. A cylindrical stainless steel base member is subjected to a barreling treatment so as to secure fine particles of an inorganic material having a Moh&#39;s hardness of greater than 9 to the slide surface of the guide post. Moreover, the surface of the guide pole can be finished by a surface rolling to reduce the number of scratch-like grooves in the slide surface having a depth of more than 0.2  mu m and a width of less than 40  mu m to less than 11 per millimeter in the sliding direction of the tape.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic tape cassette, particularly,to a magnetic tape cassette used in a VTR (video tape recorder) or thelike.

A variety of magnetic tape cassettes, including those of the Beta andVHS type, are known for use in VTRs or the like. The conventionalcassettes all employ the same basic structure. FIG. 1 shows, as anexample, a cassette of the VHS type. The magnetic tape cassette of FIG.1 includes a tape guiding structure such as cylindrical tape guides 13,a guide pole 14, a guide roller 15, and a pressure pad 16 whichcooperate to guide a magnetic tape T in running between a supply reel 11and a take-up reel 12.

The sliding characteristics of the magnetic tape on these guide membersgreatly affect the tape running characteristics. This can be readilyunderstood from the fact that the magnet tape T is manufactured byforming a magnetic layer on a flexible film support made of acetate,polyester, or vinyl chloride, as is well known in the art. To increasethe total recording and playing time of the tape, or to improve theelectromagnetic properties of the tape, a thinner tape having a highsurface smoothness has been developed.

To take advantage of the improved sliding properties of such a tape, itis necessary to improve the surface smoothness of the above guidemembers. That is, as the thickness of the magnetic tape is reduced, itsrigidity is decreased, and therefore the running characteristics of thetape are more strongly affected by the guide members. In the case of atape used for recording and reproducing data at a high recordingdensity, even slight variations in the behavior of the magnetic tape canstrongly affect the recording and reproducing operations carried outwith the tape. For example, variations in the behavior of the taperunning can result in a jitter phenomenon, making the reproduced picturewaver in the horizontal direction. Accordingly, it is essential to makethe tape run smoothly.

The inventors have conducted intensive research into the above-describedproblems and found that variations in the running characteristics of thetape are largely attributable to the guide members which are in contactwith the magnetic layer of the tape, namely, the guide pole 14 as shownin the example of FIG. 1, and to the surface characteristics of the tapeitself. For instance, a tape with a magnetic surface having an Ra valueof less than 0.01 μm (cut-off value of 0.25 mm) is more likely to haveadversely affected tape running characteristics.

Thus, the above-described drawbacks may be eliminated with the provisionof a guide pole 14 (with which the surface of the magnetic layer of thetape is brought into contact) which is made harder than a conventionalguide pole and which is polished or subjected to other surface treatmentso as to improve its surface smoothness. On the other hand, suchtechniques significantly increase the manufacturing cost of the guidepole, making the guide pole impractical for ordinary manufacturing use.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amagnetic tape cassette having a guide pole contacting the magnetic layerof the tape contained in the cassette which allows the tape to runsmoothly, but which can be manufactured at a low cost.

The foregoing and other objects of the present invention have been metby the provision of a magnetic tape cassette having a guide pole whichcontacts the magnetic surface of the tape contained in the cassette inwhich, in accordance with the invention, the surface of the guide poleis machined to a predetermined surface roughness.

For this purpose, in the magnetic tape cassette of the presentinvention, inorganic particles having a Moh's hardness of 9 or greaterare adhered to the surface of the guide pole. Also, the surface of theguide pole can be machined and finished so that the number of finegrooves of depth 0.2 μm or greater and width 40 μm or greater formed inthe surface of the guide pole and extending in the sliding direction ofthe tape on the surface of the guide pole is not more than ten permillimeter.

The nature, principle and utility of the present invention will becomemore apparent from the following detailed description read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view, with parts cut away, showing an example of aconventional magnetic tape cassette;

FIG. 2 is a perspective view showing a guide pole used in the magnetictape cassette, which guide pole is constructed in accordance with afirst embodiment of the present invention;

FIGS. 3 and 4 are diagrams showing electron microscopic photographs ofthe surface of a guide pole in a magnetic tape cassette constructed inaccordance with a first embodiment of the present invention;

FIG. 5 is a diagram showing optical microscopic photograph of thesurface of a guide pole in a magnetic tape cassette prior to surfacerolling; and

FIG. 6 is a diagram showing the same surface as that of FIG. 5 aftersurface rolling.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the accompanying drawings.

FIG. 2 shows a guide pole produced in accordance with a first embodimentof the invention. The guide pole 1 may be employed, for example, in themagnetic tape cassette 10 as shown in FIG. 1, and it has the samedimensions as the guide pole 14 seen in FIG. 1.

The guide pole 1 is manufactured by the following process:

A cylindrical stainless steel body having substantially the samedimensions as the desired guide pole is subjected to centerless grindingto roughen its surface. The centerless grinding operation machines thecylindrical stainless steel body to desired dimensions while formingfine grooves appearing as scratches in the cylindrical surface extendingin the circumferential direction. The grindstone used in the centerlessgrinding operation should generally have a roughness in a range of No.100 to No. 140. In selecting a grindstone of appropriate roughness, notonly the time required for grinding the raw cylindrical body, but alsothe grade of a barrel finishing described below should be taken intoconsideration.

Thereafter, the surface of the guide pole 1 is finished by barrelfinishing. In the barreling operation, the centerless-ground guide poleis rotated together with an abrasive such as aluminum oxide particles(spherical shape, 1 μm to 2 mm diameter) or silicon carbide particlesand ceramic balls (2.5 mm diameter) in a closed container. During thebarreling operation, the abrasive particles rub and strike againstthemselves and the guide pole 1 in the container, thus becoming smallerand smaller.

In the barreling operation, the surface of the guide pole 1 is finishedwith aluminum oxide particles to provide a considerably smooth surface2, while simultaneously the aluminum oxide particles secured themselvesto the surface 2 are implanted therein. That is, during the barrelingoperation, the rubbing of the guide pole 1 against the balls and/orother guide poles being simultaneously processed allows the aluminumoxide particles 3 to finish the surface 2 of the guide pole and alsocauses some of the aluminum oxide particles to become fixed to thesurface 2 in such a manner as to fill in minute concave portions of thesurface 2. These aluminum particles form a part of the material of theguide pole 1 and cannot be dislodged by external forces applied to thepole 1 when in use, that is, they are positively held as part of thesurface 2 of the guide pole 1. The aluminum oxide particles which securethemselves to the minute concave parts of the surface 2 of the guidepole are those which have been pulverized to a diameter less than thewidth of the minute concave parts, for example, about 3 μm.

The aluminum oxide particles 3 secured to the surface 2 of the guidepole 1 are as shown in FIGS. 3 and 4, which are diagrams depictingmicroscopic photographs taken of such a guide pole 1. The magnificationin FIG. 3 is 2,000, while in FIG. 4 the magnification is 10,000.

The distribution of the aluminum oxide particles 3 is such that, asshown in FIG. 3, each 25 μm by 20 μm area on the surface 2 containsabout 15 or 16 particles 3. The distribution and the securing of thealuminum oxide particles can be suitably controlled, for instance, byselection of the barreling time and by selection of the size of theceramic balls.

The surfaces of the aluminum oxide particles secured to the surface 2 ofthe guide pole 1 forming parts of the surface 2 of the guide pole 1 areflat, as is apparent from FIG. 4. This is due to the fact that thesurfaces of the adhering aluminum oxide particles are flattening duringthe barreling operation. The smoothness of the surface 2 may besubstantially the same as that of a conventional guide pole. Forexample, it is preferable that, with a cut-off value of 0.08 mm, the Ravalue is of the order of 0.01 to 0.05 μm.

As described above, the aluminum oxide particles, which are inorganicparticles having a Moh's hardness greater than 9, are suitablydistributed on the surface 2 of the guide pole 1 in such a manner as toform integral parts of the surface 2, and the roughness (alternately,smoothness) of the surface 2 is equal to or greater than that of aconventional guide pole. As a result, the frictional resistance betweenthe guide pole 1 and the magnetic surface of the tape is held to a smallvalue.

The reason for this is considered to be as follows: Because, the surface2 of the guide pole 1 of the present invention includes .hard particlesas described above, the effective hardness of the surface becomes thehardness of the particles, with the result that the wear resistance ofthe guide pole is significantly improved. Moreover, the effectivecontact surface with the magnetic surface of the tape is the portionswhere the tape contacts the particles, rather than the other material ofthe guide pole. Hence, the effective contact area between the magneticsurface of the tape and the guide pole is substantially reduced, andaccordingly the frictional resistance between the tape and the guidepole is reduced.

In the above-described embodiment, aluminum oxide particles are employedas the inorganic particles, but the invention is not limited to the useof this material. That is, other particles having a Moh's hardnessgreater than 9, such as silicon carbide particles, can also be used.

As described above, in the first embodiment of the present invention,inorganic particles having a Moh's hardness greater than 9 are adheredto the surface of the guide pole on which the magnetic surface of themagnetic tape contained in the cassette slides, whereby the wearresistance of the guide pole is increased and the slide resistance ofthe magnetic tape accordingly decreased. Furthermore, the use ofaluminum oxide particles or the like provides substantially the sameeffect as an oxide film, with the result that excellent tape runningcharacteristics can be maintained over long periods of use, and jitterand tape jamming substantially prevented.

The guide pole according to the present invention can be manufacturedwith a very simple manufacturing process such as a barreling method. Asa result, a magnetic tape cassette having excellent data recording andreproducing characteristics can be obtained at a low cost with the useof the invention.

Conducive to a complete understanding of the present invention, specificexamples thereof will be described.

EXAMPLE 1

VHS magnetic tape cassettes, "S-MASTER", manufactured by Fuji Photo FilmCo., Ltd., were used for the tests.

Guide poles having a cylindrical shape with a diameter of 2.5 mm, asshown in FIG. 3, were formed as follows: A cylindrical stainless steelbody of appropriate shape containing 65 to 75% Fe, 15 to 20% Cr, and 10to 15% Ni by weight was provided, and aluminum oxide particles, siliconcarbide particles, and Cr₂ O₃ particles were used as the inorganicparticles having a Moh's hardness of 9 or higher. The average particlesize of the inorganic particles was in a range of 0.1 μm to 0.5 μm. Thedistribution of the inorganic particles was such that, similar to thecase of FIG. 3 discussed above, five to 30 particles were present ineach 25 μm by 20 μm area. Moreover, also similar to the above-describedembodiment of the present invention, the inorganic particles wereadhered to the surfaces of the guide poles by barrel finishing.

With respect to the smoothness of the surfaces of the guide poles, theRa value was in a range of 0.03 μm to 0.04 μm, with the cut-off valuebeing 0.08 mm.

As a comparison, guide poles were produced using inorganic particleshaving a Moh's hardness of 8 or less.

The results of the evaluation of the magnetic tape cassettes of theabove examples are listed in Table 1 below:

                  TABLE 1                                                         ______________________________________                                                                          Evaluation                                  Sample     Inorganic   Moh's      (Occurrence                                 No.        Particles   Hardness   of Jitter)                                  ______________________________________                                        1          aluminum oxide                                                                            9          OO                                                     (Al.sub.2 O.sub.3)                                                 2          ilicon carbide                                                                            9          O                                                      (SiO)                                                              3          topaz       8          X                                           (Compar.)  (SiO.sub.4)                                                        4          quartz      7          XX                                          (Compar.)  (SiO.sub.2)                                                        ______________________________________                                    

In Table 1, the evaluation was carried out as follows: With thetemperature held at 5° C and the relative humidity at 80%, a datarecording and reproducing device, Model VTRG-21, manufactured byMatsushita Denki Co., Ltd., and a jitter meter, Model MK-611A, producedby Meguro Dempa Co., were used to measure the amount of jitter. When theamount of jitter was less than 0.10 μsec., such is indicated in Table 1by "OO", when the amount of jitter was larger than 0.10 μsec.(inclusive) and smaller than 0.15 μsec., an indication of "O" isemployed, when the amount of jitter was larger than 0.15 μsec.(inclusive) and smaller than 0.20 μsec., such is shown by a mark "X",and when larger than 0.20 μsec. (inclusive), a mark of "XX" is shown.

As is apparent from Table 1, in the case where the inorganic particleshad a Moh's hardness of 9 or higher, the resultant picture reproducedfrom the tape was substantially free from jitter. On the other hand, asthe Moh's hardness was reduced to less than 9, jitter appeared. That is,the effect is significantly different when particles having a Moh'shardness of 9 or greater were employed.

A second embodiment of the invention will now be described.

In the second embodiment of the present invention, the guide pole issubjected to centerless grinding and barrel finishing, similar. to thecase of the first-described embodiment of the present invention.Thereafter, the guide pole is subjected to surface rolling. The smallconvex parts remaining on the surface of the surface of the barreledguide pole are flattened by the surface rolling operation. This surfaceflattening operation is carried out beginning with the smallest of thefine groove-like scratches formed in the centerless grinding operationand which are left after the barrel finishing.

FIG. 5 shows the surface 2 of the guide pole 1 prior to surface rolling,and FIG. 6 shows the same surface after surface rolling. In FIGS. 5 and6, the optical microscopic photographic magnification is 200. As isapparent from FIGS. 5 and 6, the number of scratch-like grooves 5 in thesurface 2 of the guide pole is considerable reduced by barrel finishing;that is, the surface 2 is made considerably smooth.

The roughness of the surface 2 after being subjected to barrel finishingwas measured by moving a measuring stylus along the guide pole in thelongitudinal direction so as to measure the surface roughness in termsof up and down movement of the stylus.

In the second embodiment of the present invention, the surface 2 of theguide pole 1 is barreled and surface finished until the .number ofscratch-like grooves 5 having a depth of more than 0.2 μm and width ofless than 40 μm was ten or less per millimeter in the longitudinaldirection of the guide pole 1, that is, in the widthwise direction ofthe magnetic tape. If the number of grooves 5 is more than ten permillimeter, then, when the guide pole is used with a magnetic tapehaving a magnetic surface with a smoothness of less than 0.01 μm, thetape running behavior becomes unstable, as a result of which jitter canfrequently occur.

If, on the other hand, the surface 2 of the guide pole 1 has a mirrorfinish, the a magnetic tape having a very smooth magnetic surface willtend to stick to the surface of the guide pole, thus greatly increasingthe amount of frictional resistance and degrading the tape runningcharacteristics.

As described above, in accordance with the second embodiment of thepresent invention, the surface of the guide pole which is in slidingcontact with the magnetic surface of the magnetic tape is formed in sucha manner that the number of scratch-like grooves having a depth of morethan 0.2 μm and a width of less than 40 μm in the direction of slidingof the magnetic tape is not more than ten per millimeter. As a result,in the magnetic tape cassette of the present invention, the magnetictape is substantially free, for example, from oscillatory movementcaused by the surface roughness of the guide pole. Hence, the magnetictape cassette provides excellent recording and reproductioncharacteristics being free, for example, from jitter.

Conducive to a full understanding of this embodiment of the presentinvention, specific examples of the second embodiment will be described.

EXAMPLE 2

VHS magnetic tape cassettes, "S-MASTER", manufactured by Fuji Photo FilmCo., Ltd., and magnetic tapes whose surface Ra value (cut-off value 0.25mm) was 0.009 μm were used for the tests.

Cylindrical guide poles as shown at 2 in the above-discussed drawingsand having a diameter of 2.5 mm were formed by the following process: Astainless steel body of appropriate shape containing 65 to 75% Fe, 15 to20% Cr, and 10 to 15% Ni by weight was provided. Grindstones used incenterless grinding of the guide poles had a roughness of No. 120. Theabrasives used in the barrel finishing of the guide poles were aluminumoxide particles having an average particle size of 1.5 mm.

A plurality of guide poles were manufactured under these specificationswith processing conditions such as barrel finishing time varied. Allspecimens were surface rolled. The guide poles thus manufactured weremeasured for scratch-like grooves in the sliding direction of themagnetic tape, and observations of picture jitter for each were carriedout.

The specimens were measured and a surface roughness pattern chart formedby measuring a predetermined part of length 1.6 mm with a constantstylus pressure of 0.4 gf with a diameter of 5 μm and measuring speed of0.06 mm/sec, with a chart speed being set at 15 mm/sec and with amagnification of 50,000:1 in the direction of the depth.

As a result, it was found that almost all specimens had fine groove lessthan 0.2 μm in depth, which is advantageous since jitter arisesprimarily for the case where the grooves are deeper than 0.2 μm and havea width of less than 40 μm.

The results of these measurements are presented in Table 2 below:

                  TABLE 2                                                         ______________________________________                                        Specimen                                                                              Grooves   Groove     Groove  Evaluation                               No.     per mm    Depth (μm)                                                                            Width (μm)                                                                         (Jitter)                                 ______________________________________                                        1       2         0.2        20      00                                       2       5         0.2        20      00                                       3       6         0.2        50      X                                        4       9         0.4        40      0                                        5       10        0.2        40      0                                        6       13        0.2        20      X                                        7       20        25         50      X                                        ______________________________________                                    

In Table 1, the evaluation was carried out as follows: With thetemperature held at 5° C and the relative humidity at 80%, a datarecording and reproducing device, Model VTRG-21, manufactured byMatsushita Denki Co., Ltd., and a jitter meter, Model MK-611A, producedby Meguro Dempa Co., were use to measure the amount of jitter. When theamount of jitter was less than 0.10 μsec., such is indicated in Table 1by "OO", when the amount of jitter was larger than 0.10 μsec.(inclusive) and smaller than 0.15 μsec., an indication of "O" isemployed, when the amount of jitter was larger than 0.15 μsec.(inclusive) and smaller than 0.20 μsec., such is shown by a mark "X",and when larger than 0.20 μsec. (inclusive), a mark of "XX" is shown.

As is apparent from Table 2, in the case where the number of finegrooves having a depth larger than 0.2 μm and width of less than 40 μmwas ten or less, no jitter was observed, and hence the resultant picturewas satisfactory.

As described above, the invention provides a magnetic tape cassettehaving a guide pole which is in sliding contact with the magneticsurface of the tape contained in the cassette having properties ensuringthat the magnetic tape has excellent running characteristics, preventingthe occurrence of jitter and tape jamming and being relatively low inmanufacturing cost.

What is claimed is:
 1. In a magnetic tape cassette containing a magnetictape and having a metal guide pole which guides said magnetic tape whilein sliding contact with a magnetic surface of said tape, the improvementwherein inorganic particles having a Moh's hardness of 9 or higher frompat of a surface of said guide pole in sliding contact with saidmagnetic surface of said tape.
 2. The magnetic tape cassette of claim 1,wherein said inorganic particles are made of aluminum oxide.
 3. Themagnetic tape cassette of claim 1, wherein said inorganic particles aremade of silicon carbide.
 4. The magnetic tape cassette of claim 1,wherein a base portion of said guide pole is made of stainless steel. 5.In a magnetic tape cassette containing a magnetic tape and having ametal guide pole which guides said magnetic tape while in slidingcontact with a magnetic surface of said tape, the improvement wherein anumber of fine scratch-like grooves in a surface of said guide pole insliding contact with said magnetic tape having a depth of 0.2 μm or moreand a width of 40 μm or less in a sliding direction of said magnetictape on said surface is less than 11 per millimeter.